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Distribution limits of a host race of the goldenrod gallfly: Are cold winters good?
Whipple, Amy*,1, Young, Lauren2, Heinrich, Paul, Ferry, Jake1, Urian, Alyson1, Pardus, Jessica1, Abrahamson, Warren1, 1 Bucknell University, Lewisburg, PA2 University of Nebraska, Lincoln, NE
ABSTRACT- The goldenrod gallfly is distributed throughout the eastern United States and beyond, but one of two host races is restricted to a more northerly range. The range of the host race on Solidago gigantea ('gigantea' flies) is more confined than the range of their host plant. We used collection site data and modeled climate data (from The Climate Source) to distinguish among three distinct hypotheses as to the mechanism that restricts the range of gigantea flies. The hypothesized restrictions are: 1) the timing of fly emergence relative to plant growth in the spring, 2) costs of summer heat shock, and 3) energy loss during over-wintering. Hypotheses 2 and 3 require that energetic costs have a greater effect on the ability of the gigantea host race to maintain a population since the other host race extends further south. This is likely because gigantea flies are smaller and have lower gall-induction rates, lower levels of intraspecific competition, and smaller population sizes than the widespread host race. Here we present results demonstrating that gigantea flies also have fewer eggs than the other host race. This smaller egg reserve may play a role in translating individual energetic costs into greater population consequences. Classification tree analysis of the distribution data found that, beyond host presence, average maximum daily temperatures for December best explained the presence of gigantea flies. Thus, the over-wintering temperature hypothesis is supported.
KEY WORDS: Eurosta solidaginis, GIS, distribution, host race